Cathode current collector for electrical energy storage device and method for manufacturing the same

ABSTRACT

Disclosed is a cathode current collector for an electrical energy storage device and a method for manufacturing the same, which improves adhesion between a current collector and an electrode material and provide a high reaction surface area, thereby improving the performance of the electrical energy storage. In particular, a first alumina film is formed on the surface of an aluminum foil using an anodic oxidation process. Next, the first alumina film formed on a surface of the aluminum foil is removed through etching and a second alumina film is formed on the surface of the aluminum foil, from which the first alumina film is removed, using the anodic oxidation process again. Subsequently, a carbon layer is coated on a surface of the aluminum foil on which the second alumina film is formed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims under 35 U.S.C. §119(a) the benefit of KoreanPatent Application No. 10-2011-0107558 filed Oct. 20, 2011, the entirecontents of which are incorporated herein by reference.

BACKGROUND

(a) Technical Field

The present invention relates to a cathode current collector for anelectrical energy storage device. More particularly, it relates to acathode current collector for an electrical energy storage device and amethod for manufacturing the same, which can improve the adhesionbetween a current collector and an electrode material and provide a highreaction surface area, thereby improving the performance of theelectrical energy storage.

(b) Background Art

An electrical energy storage device is a device that generateselectrical energy between electrodes. Typically, an electrode is formedby coating an electrode slurry on a current collector, and the electrodeslurry is prepared by mixing an electrode material, which includes anelectrode active material for storing energy, a conductive material forimparting electrical conductivity, and a binder for binding theconductive material to the current collector and providing a bondingforce between them, with a solvent.

The current collector functions to accumulate electrons generated by anelectrochemical reaction of the active material and transfers theelectrons to an external circuit in the electrical energy storage devicesuch as a battery, an electrochemical capacitor, etc. In order toaccommodate the electrons produced in the active material as much aspossible, the current collector must be strongly bonded to the activematerial and have a higher contact area. Moreover, the current collectoris required to have high electrical conductivity so as to smoothlytransfer the electrons emitted from the active material to the externalcircuit.

However, the electrode material and the current collector inconventional electrodes have a limited contact area, and thus theadhesion between the electrode material and the current collector isreduced. As a result, the electrode material attached to the surface ofthe current collector is removed during operation of the electricalenergy storage device. The reduction in the adhesion and the removal ofthe electrode material increases the internal resistance of theelectrical energy storage device, which degrades the outputcharacteristics and reduces the charge capacity, thereby significantlydegrading the performance of the electrical energy storage device.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY OF THE DISCLOSURE

The present invention provides a cathode current collector for anelectrical energy storage device and a method for manufacturing thesame, which can improve the surface structure of the cathode currentcollector by an anodic oxidation process to increase the reactionsurface area. Furthermore, the cathode current collector of the presentinvention improves the electrical conductivity and the adhesion with acathode active material by coating a conductive carbon layer on thesurface of the cathode current collector.

In one aspect, the present invention provides a method for manufacturinga cathode current collector, the method comprising: forming a firstalumina film on the surface of an aluminum foil by an anodic oxidationprocess; removing the first alumina film formed on the surface of thealuminum foil by etching; forming a second alumina film on the surfaceof the aluminum foil, from which the first alumina film is removed, bythe anodic oxidation process; and coating a carbon layer on the surfaceof the aluminum foil on which the second alumina film is formed.

In an exemplary embodiment, the method may further comprise, prior toforming the second alumina film, washing the aluminum foil, from whichthe first alumina film is removed, with deionized water and drying theresulting aluminum foil in a preheated oven.

In another aspect, the present invention provides a cathode currentcollector for an electrical energy storage device, the cathode currentcollector comprising: an aluminum foil layer having a high specificsurface area on which an alumina film having a nanoporous structure isformed; and a conductive carbon layer coated on the surface of thealuminum foil layer.

In still another aspect, the present invention provides a method formanufacturing a cathode for an electrical energy storage device, themethod comprising: forming a first alumina film on the surface of analuminum foil by an anodic oxidation process; removing the first aluminafilm formed on the surface of the aluminum foil by etching; forming asecond alumina film on the surface of the aluminum foil, from which thefirst alumina film is removed, by the anodic oxidation process; coatinga carbon layer on the surface of the aluminum foil on which the secondalumina film is formed; and coating a cathode slurry on the surface ofthe carbon layer.

In yet another aspect, the present invention provides a cathode for anelectrical energy storage device, the cathode comprising: a cathodecurrent collector including an aluminum foil layer having a highspecific surface area, on which an alumina film having a nanoporousstructure is formed, and a conductive carbon layer coated on the surfaceof the aluminum foil layer; and a cathode material layer coated on thesurface of the cathode current collector. More specifically, the cathodematerial layer coated on the surface of the cathode current collectorincludes a cathode active material, a conductive material, and a binder.

In still yet another aspect, the present invention provides anelectrical energy storage device including a cathode having a cathodecurrent collector with an aluminum foil layer having a high specificsurface area, on which an alumina film having a nanoporous structure isformed, and a conductive carbon layer coated on the surface of thealuminum foil layer; and a cathode material layer coated on the surfaceof the cathode current collector where the cathode material layerincludes a cathode active material, a conductive material, and a binder.

Other aspects and exemplary embodiments of the invention are discussedinfra.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now bedescribed in detail with reference to certain exemplary embodimentsthereof illustrated the accompanying drawings which are givenhereinbelow by way of illustration only, and thus are not limitative ofthe present invention, and wherein:

FIG. 1 is schematic diagram showing an anodic oxidation process using anelectrochemical device;

FIG. 2 is a schematic diagram sequentially showing the steps of a methodfor manufacturing a cathode current collector for an electrical energystorage device in accordance with an illustrative embodiment of thepresent invention, in which an aluminum current collector having a highspecific surface area is manufactured by an aluminum anodic oxide (AAO)process;

FIG. 3 is a schematic diagram showing the calculation of a gain insurface area due to a nanoporous structure formed on the surface of acathode current collector in accordance with the illustrative embodimentof the present invention;

FIG. 4 is a schematic diagram showing the specific surface area of acathode current collector in accordance with the illustrative embodimentof the present invention and that of an existing cathode currentcollector; and

FIG. 5 is a schematic diagram showing the cross-sectional structures ofa cathode current collector and a cathode in accordance with theillustrative embodiment of the present invention.

Reference numerals set forth in the Drawings includes reference to thefollowing elements as further discussed below:

-   -   1: electrolyzer    -   2: anode electrode    -   3: aluminum foil    -   4: counter electrode    -   5: platinum foil    -   6: electrolyte    -   7: stirrer    -   10: aluminum foil layer    -   11: alumina film    -   12: carbon layer    -   13: cathode current collector    -   14: cathode material layer    -   15: cathode

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of theinvention. The specific design features of the present invention asdisclosed herein, including, for example, specific dimensions,orientations, locations, and shapes will be determined in part by theparticular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent partsof the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Hereinafter reference will now be made in detail to various embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings and described below. While the invention will bedescribed in conjunction with exemplary embodiments, it will beunderstood that present description is not intended to limit theinvention to those exemplary embodiments. On the contrary, the inventionis intended to cover not only the exemplary embodiments, but alsovarious alternatives, modifications, equivalents and other embodiments,which may be included within the spirit and scope of the invention asdefined by the appended claims.

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g., fuels derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example bothgasoline-powered and electric-powered vehicles.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. As used herein, the term “and/or”includes any and all combinations of one or more of the associatedlisted items.

The present invention provides a cathode current collector and a methodfor manufacturing the same, which improves the surface structure of thecathode current collector to increase the reaction surface area, inwhich a nano-honeycomb structure is formed on the surface of the currentcollector by an anodic oxidation process such as an aluminum anodicoxide (AAO) process, thereby increasing the specific surface area.

When the surface area of the current collector increases, the surfacearea to which an electrode material is attached also increases, which inturn improves the adhesion between the electrode material and thecurrent collector and further increases the surface area of theelectrode material, thereby increasing the reaction surface area of theelectrical energy storage device. As such, when the surface area of thecurrent collector increases, it is possible to increase the contact areabetween the electrode material and the current collector withoutchanging components of the electrode material, thereby increasing thereaction surface area.

When the anodic oxidation process is used, it is possible to control thepore size and depth on the surface of the current collector by changingthe time and conditions of the anodic oxidation, and thus it is possibleto control the pore size of the nano-honeycomb structure from severalnanometers to several hundred nanometers and the pore depth to severalhundred micrometers with low-cost equipment.

According to the illustrative embodiment of the present invention, analumina film having a high specific surface area with a nanoporousstructure (e.g., nano-honeycomb structure) is formed on the surface ofthe current collector by anodically oxidizing an aluminum currentcollector, and the surface of the alumina film is coated with aconductive carbon material, thereby manufacturing a cathode currentcollector.

A carbon layer formed on the surface of the alumina film improves theelectrical conductivity of the current collector, prevents the aluminumcurrent collector from being damaged by an electrolyte of the electricalenergy storage device, and improves adhesion with a cathode material.

FIG. 1 is schematic diagram showing an anodic oxidation process using anelectrochemical device. An aluminum foil 3 to be anodically oxidized isconnected to an anode electrode 2 in an electrolyzer 1 containing anelectrolyte 6 such as oxalic acid, phosphoric acid, sulfuric acid, etc.,and a platinum foil 5 is connected to a counter electrode 4.

The electrolyte 6 is continuously stirred with a stirrer 7 to prevent alocal increase in the concentration of the electrolyte, and the aluminumfoil 3 is anodically oxidized by electrolysis, thereby forming analumina film having a hexagonal structure such as a nano-honeycombstructure on the surface of the aluminum foil 3. The alumina film formedon the surface of the aluminum foil 3 is a porous film having ahoneycomb structure with a plurality of pores, and the pore size anddepth of the alumina film may be controlled by changing the etchingconditions of the alumina film. Otherwise, the size and the distancebetween the pores may be controlled by the type of the electrolyte. Thatis, the nanoporous structure formed on the surface of the aluminum foilduring anodic oxidation may be controlled by the type of the electrolyteand/or by the change of the etching conditions.

Typically, the size and depth of the pores formed on the surface of thealuminum foil during anodic oxidation are proportional to the length oftime the anodic oxidation process is applied and the distance betweenthe pores is proportional to the voltage applied during the anodicoxidation process.

In the illustrative embodiment of the present invention, the pore sizeand depth on the surface of the aluminum foil, i.e., the currentcollector, are proportional to the time of a second anodic oxidationprocess. In detail, during manufacturing the anodic current collector ofthe present invention, the alumina film formed during the first anodicoxidation process has an irregular porous structure. Thus, the firstformed alumina film is removed by an etching process, and thus a secondanodic oxidation process is performed to form a uniform nanoporoussecond alumina film on the surface of the current collector.

That is, in order to form a uniform nanoporous surface structure on thecurrent collector, the alumina film formed by the first anodic oxidationprocess is removed by the etching process, and the second anodicoxidation process is performed to form a nano-honeycomb structure havinga regular and uniform surface. Therefore, in the illustrative embodimentof the present invention, the pore size and the surface depth of thepores on the current collector are proportional to the length of timethe second anodic oxidation process is applied.

After the anodic oxidation process, a non-conductive alumina film isformed on the surface of the current collector, and thus a carbonmaterial having a high specific surface area is coated on the surface ofthe current collector using a plasma coater to form a conductive carbonlayer, thereby providing electrical conductivity. The carbon layer onthe surface of the current collector provides electrical conductivity tothe current collector, prevents corrosion of the current collector, andimproves adhesion with the electrode material.

Referring to FIG. 5, the anodic current collector formed by theabove-described processes comprises an aluminum foil layer 10 having ahigh specific surface area, on which an alumina film 11 having anano-honeycomb structure is formed, and a carbon layer 12 having a highspecific surface area and coated on the surface of the aluminum foillayer 10.

Next, the following examples illustrate the invention and are notintended to limit the same.

Example 1 Manufacture of Anodic Current Collector

First, an anodic oxidation process was performed using anelectrochemical device as shown in FIG. 1.

An aluminum foil was connected to an anode electrode (i.e., a positiveelectrode) of the electrochemical device, and a platinum foil wasconnected to a counter electrode. In order to maintain the potentialbetween the two electrodes at a constant level, the distance between thealuminum foil and the platinum foil was maintained at 50 mm A constantvoltage was supplied from a power supply between the two electrodes toinduce an anodic oxidation reaction at a constant temperature, and anelectrolyte was continuously stirred with a stirrer to prevent a localincrease in the concentration of the electrolyte.

A 1 M phosphoric acid solution was used as the electrolyte in a firstanodic oxidation process, and a voltage of 40 V was applied between thetwo electrodes to induce electrolysis such that the surface of thealuminum foil was oxidized, thereby forming a first alumina film. Then,the first alumina film formed on the surface of the aluminum foil wasremoved by etching using a mixed solution of chromic acid and phosphoricacid at 65° C., thereby forming a uniform surface structure. Here, themixed solution was prepared by mixing 1.8 wt % of chromic acid, 6 wt %of phosphoric acid, and 92.2 wt % of HgCl₂. Subsequently, the resultingaluminum foil was washed with deionized water for about 15 minutes anddried in a preheated oven at 60° C. for about 1 hour.

In a second anodic oxidation process, a 1 M phosphoric acid solution wasused as the electrolyte, and a voltage of 165 V was applied between thetwo electrodes to perform electrolysis for about 100 minutes such thatthe surface of the aluminum foil, from which the first alumina film wasremoved, was oxidized, thereby forming a second alumina film. Here, thetemperature of the electrolyzer was maintained at 2° C. The secondalumina film formed on the surface of the aluminum foil by theabove-described process had an aluminum anodic oxide structure in whichpores had a diameter of 100 nm and a depth of 3 μm and the distancebetween the pores was about 180 nm.

A conductive carbon layer was coated on the surface of the aluminum foilhaving the aluminum anodic oxide structure (i.e., the alumina film)using a plasma coater. Graphite having a high specific surface area wasused as a carbon source, and a high voltage was applied between theanodically oxidized current collector (i.e., the aluminum foil) and atarget to produce carbon plasma, thereby forming the carbon layer with athickness of several nm on the surface of the current collector.

Example 1 Manufacture of Cathode

A cathode slurry (or cathode material) was prepared by mixing 60 wt % ofsulfur having a particle size of 100 mesh as a cathode active material,20 wt % of Super C as a conductive material, and 20 wt % ofpolyvinyliden fluoride (PVDF) as a binder. The prepared cathode slurrywas coated on the surface of the cathode current collector (on which thecarbon layer was coated) formed in Example 1 to form a cathode. Theprocess of manufacturing the cathode including the process manufacturingof the cathode current collector as described in Examples 1 and 2 can beshown in FIG. 2.

Example 3 Manufacture of Lithium-Sulfur Battery

The cathode formed in Example 2 was used as a cathode, and a liquidelectrolyte prepared by dissolving LiTFSI with a concentration of 1 M inTEGDME/DIOX mixed in a volume ratio of 5:5 was used as an electrolyte. Alithium-sulfur battery was manufactured using a lithium foil cathodewith a thickness of 200 μm. As a result of a peel-off test, the adhesionbetween the cathode current collector of Example 1 and the cathodeslurry (i.e., the cathode material) was higher than that of a typicalcurrent collector made of aluminum foil.

The surface area of the current collector manufactured in the abovemanner is increased to in turn increase the reaction area (see FIG. 4),which in turn improves the adhesion between the electrode material andthe current collector, thereby improving the performance of theelectrical energy storage device.

As shown in FIG. 5, the cathode 15 manufactured by the above-describedprocess includes: a cathode current collector 13 including an aluminumfoil layer 10 having a high specific surface area, on which an aluminafilm 11 having a nanoporous structure is formed, and a conductive carbonlayer 12 having a high specific surface area and coated on the surfaceof the aluminum foil layer 10; and a cathode material layer 14 coated onthe surface of the cathode current collector 13. This cathode materiallayer 14 includes a cathode active material, a conductive material, anda binder.

Moreover, the electrical energy storage device manufactured by theabove-described process includes a first cathode 15 having a cathodecurrent collector 13, which includes an aluminum foil layer 10 having ahigh specific surface area, on which an alumina film 11 having ananoporous structure is formed, and a conductive carbon layer 12 havinga high specific surface area and coated on the surface of the aluminumfoil layer 10, and a cathode material layer 14 coated on the surface ofthe cathode current collector 13. Again the cathode material layerincludes a cathode active material, a conductive material, and a binder.The electrical energy storage device also includes a second cathode (notshown) formed of a cathode slurry containing a cathode active material;and an electrolyte.

Meanwhile, in the case of the aluminum anodic oxide structure formed onthe surface of the cathode current collector, a gain in surface area isachieved depending on the pore size and depth of the alumina film, whichcan be calculated using a simple schematic model shown in FIG. 3. If thesurface area of the current collector before the aluminum anodic oxideprocess is A₀ and if a plurality of square pores with a length of d anda height of H are formed at an interval of d on the surface of thecurrent collector, the surface area A can be calculated as follows:

Surface area A=A ₀*(1+H/d)

Therefore, if a plurality of pores with a diameter of 100 nm and a depthof 3 μm are formed at an interval of 180 nm on the surface of thecurrent collector, the gain in surface area due to the anodic oxidationreaches about 31 times.

As described above, the method for manufacturing the cathode currentcollector according to the present invention increases the specificsurface area of the cathode current collector to in turn increase thecontact area between the current collector and the electrode material,which in turn improves the adhesion between the electrode material andthe current collector and further provides a high reaction surface area,thereby improving the performance of the electrical energy storage.Moreover, the cathode current collector according to the presentinvention increases electrical conductivity by the carbon layer coatedon the surface thereof, prevents corrosion of the aluminum currentcollector due to the electrolyte, and improves adhesion with theelectrode material.

The invention has been described in detail with reference to exemplaryembodiments thereof. However, it will be appreciated by those skilled inthe art that changes may be made in these embodiments without departingfrom the principles and spirit of the invention, the scope of which isdefined in the appended claims and their equivalents.

What is claimed is:
 1. A method for manufacturing a cathode currentcollector, the method comprising: forming a first alumina film on asurface of an aluminum foil using an anodic oxidation process; removingthe first alumina film formed on the surface of the aluminum foilthrough etching; forming a second alumina film on a surface of thealuminum foil, from which the first alumina film is removed, using theanodic oxidation process; and coating a carbon layer on a surface of thealuminum foil on which the second alumina film is formed.
 2. The methodof claim 1, further comprising, prior to forming of the second aluminafilm, washing the aluminum foil, from which the first alumina film isremoved, with deionized water and drying the resulting aluminum foil ina preheated oven.
 3. A cathode current collector for an electricalenergy storage device, the cathode current collector comprising: analuminum foil layer having a high specific surface area on which analumina film having a nanoporous structure is formed; and a conductivecarbon layer coated on a surface of the aluminum foil layer.
 4. Thecathode current collector of claim 3 wherein the nanoporous structure isa nano-honeycomb structure
 5. A method for manufacturing a cathode foran electrical energy storage device, the method comprising: forming afirst alumina film on a surface of an aluminum foil using an anodicoxidation process; removing the first alumina film formed on the surfaceof the aluminum foil by etching; forming a second alumina film on thesurface of the aluminum foil, from which the first alumina film isremoved, using the anodic oxidation process; coating a carbon layer onthe surface of the aluminum foil on which the second alumina film isformed; and coating a cathode slurry on the surface of the carbon layer.6. A cathode for an electrical energy storage device, the cathodecomprising: a cathode current collector including an aluminum foil layerhaving a high specific surface area, on which an alumina film having ananoporous structure is formed, and a conductive carbon layer coated ona surface of the aluminum foil layer; and a cathode material layercoated on a surface of the cathode current collector, the cathodematerial layer including a cathode active material, a conductivematerial, and a binder.
 7. The cathode current collector of claim 6,wherein the nanoporous structure is a nano-honeycomb structure.
 8. Anelectrical energy storage device comprising a cathode, the cathodeincluding: a cathode current collector having an aluminum foil layerwith a high specific surface area, on which an alumina film with ananoporous structure is formed, and a conductive carbon layer coated ona surface of the aluminum foil layer; and a cathode material layercoated on a surface of the cathode current collector, the cathodematerial layer including a cathode active material, a conductivematerial, and a binder.
 9. The electrical energy storage device of claim8, wherein the nanoporous structure is a nano-honeycomb structure.